Absorption of weak acids

Increasing the gastric pH, which causes a decrease in absorption of weakly acidic drugs and results in a decreased drug effect (eg, digoxin, phenytoin, chlorpromazine, and isoniazid)... 

Gastric acidity will also affect the absorption of medication. An acidic environment increases the absorption of weak acids, whereas the absorption of weak bases is facilitated by a less acidic environment. Many psychotropic agents such as tricyclic antidepressants (TCAs] and benzodiazepines are weak bases. Older studies of gastric acid secretion found that women have approximately 33%-40% lower basal gastric acid secretion than do men [Yonkers and Hamilton 1995]. Gastric acid secretion may be further decreased in the luteal phase of the menstrual cycle (Booth et al. 1957]. 

As important as this pH effect is, it can be subordinated by other factors. For example, as indicated earlier, the absorptive area that a drug is exposed to can be a predominating factor. In this context, even though the acidic environment of the stomach favors the absorption of weak acids (e.g., acetylsalicylic acid), aspirin is still absorbed to a greater extent, in totality, in the small intestine. A partial list of the pH of several body compartments is shown in Table 2.4. The fact that there are some variations suggests that the disposition of some drugs may be differentially affected. 

Oral absorption depends partially on the pH of the gastrointestinal tract which is known to vary between species, as shown in table 5,3. Clearly, therefore, considerable differences in the absorption of weak acids from the stomach may occur between species. Similarly, differences might be seen in compounds which are susceptible to the acidic conditions... 

Process Licensors. Some of the well-known nitric acid technology licensors are fisted in Table 3. Espindesa, Grande Paroisse, Humphreys and Glasgow, Rhfyne Poulenc, Uhde, and Weatherly are all reported to be licensors of weak acid technology. Most weak acid plant licensors offer extended absorption for NO abatement. Espindesa, Rhfyne Poulenc, Weatherly, and Uhde are also reported (53,57) to offer selective catalytic reduction (SCR) technology. 

Rate measurements are straightforward if the carbenes can be monitored directly. As a rule, the decay of carbene absorption is (pseudo) first-order, due to rearrangement and/or reaction with the solvent. In the presence of a quencher, the decay is accelerated (Eq. 1), and the rate constant kq is obtained from a plot of k0bs versus [Q], Curved plots were often observed with proton donors (HX) as quenchers, particularly for high concentrations of weakly acidic alcohols. Although these effects have been attributed to oligomerization of the alcohols,91 the interpretation of curved plots remains a matter of dispute.76 Therefore, the rate constants reported in Tables 2-4 are taken from linear (regions of) obs-HX plots, or refer to a specified concentration of HX. 

The human body has a number of different pH environments. For example, blood plasma has a rigorously controlled pH of 7.4 (see Box 4.9), the gastric juice is usually strongly acidic (pH from about 1 to 7), and urine can vary from about 4.8 to 7.5. It is possible to predict the qualitative effect of pH changes on the distribution of weakly acidic and basic dmgs, especially in relation to gastric absorption and renal excretion ... 

Excretion of drugs will be affected by the pH of the urine. If the urine is acidic, weak bases are ionized and there will be poor re-absorption. With basic urine, weak bases are non-ionized and there is more re-absorption. The pH of the urine can be artificially changed in the range 5-8.5 oral administration of sodium bicarbonate (NaHCOs) increases pH values, whereas ammonium chloride (NH4CI) lowers them. Thus, urinary acidification will accelerate the excretion of weak bases and retard the excretion of weak acids. Making the urine alkaline will facilitate the excretion of weak acids and retard that of weak bases. 

Although transfer of drugs across the intestinal wall can occur by facilitated transport, active transport, en-docytosis, and filtration, the predominant process for most drugs is diffusion. Thus, the pK of the drug and the pH of the intestinal fluid (pH 5) will strongly influence the rate of drug absorption. While weak acids like phenobarbital (pK 7.4) can be absorbed from the stomach, they are more readily absorbed from the small intestine because of the latter s extensive surface area. 

The sodium or potassium salts of weak acids have higher absorption rate than those of acids themselves. For example, the biological activity of sodium salt of novobiocin (weak acid) is twice as compared to its calcium salt, and about 50 times larger in comparison to its free acid. 

Lower acidic environment increases absorption of weak bases (e.g., TCAs, BZDs, some antipsychotics). 

Free pyrimidines and purines are weakly basic compounds and are thus called bases. They have a variety of chemical properties that affect the structure, and ultimately the function, of nucleic acids. The purines and pyrimidines common in DNA and RNA are highly conjugated molecules (Fig. 8-2), a property with important consequences for the structure, electron distribution, and fight absorption of nucleic acids. Resonance among atoms in the ring gives most of the bonds partial double-bond character. One result is that pyrimidines are planar molecules purines are very nearly... 

The absorption spectrum of 0-1N aqueous solutions of arsenious oxide differs from that of aqueous solutions of alkali arsenites.11 This is characteristic of weak acids, the un-ionised molecules of which appear to be capable of absorbing more light than ionised molecules there is little or no difference in the absorption spectrum of a strong acid and its salts. 

The weak acid produced by condensation of the reaction gases is approximately 42% wt. nitric acid, with 58% water. The 5531 kg of weak acid produced is sent to an appropriate tray in the absorption column. 

Gastric pH is higher in newborns (pH 6-8) than in adults (pH 1-3), thus causing differences in ionization and absorption of certain chemicals (Radde, 1985). Adult levels of gastric acid production are reached at about two years of age. The alkaline gastric pH in newborns and infants may lead to enhanced bioavailability of weakly basic compounds but reduced bioavailability of weakly acidic compounds (Alcorn McNamara, 2003). 

Of relevance to the reaction of acids with wool is the demonstration of Larose (1953, 1955) that wool absorbs large amounts of dry HCl which he attributes to the binding of un-ionized molecules at the peptide group. Larose and Donovan (1956) suggest that absorption of undissociated acid also contributes to acid uptake from aqueous solutions. Speakman and Stott (1934) made a similar proposal to account for the high capacity of wool for some weak acids. 

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